Who do you think was the busiest mangaka in the history of Japanese manga (meaning, I suppose, in the history of the entire world)?

Tezuka or Ishimori, Yokoyama or Akatsuka or Mizuki?

I haven't confirmed this, but if the question is who had the most serials running at the same time, I think the answer would be either Kajiwara Ikki or Koike Kazuo. But neither of them were drawing the pictures...

If we narrow it down to "mangaka," although it's only my opinion, I think the answer might be Go Nagai.

The year is 1972. At that time, Go Nagai had the following serials running.

He went straight from Harenchi Gakuen (Shameless Institute) to Mazinger Z, without resting for a single issue. At the same time, in Magazine:

Omorai-kun: Ran until Weekly Shonen Magazine Issue 24

Devilman: Ran from Issue 25 of Weekly Shonen Magazine

Here, too, there was no break between issues. Simultaneously, in Sunday:

Kedaman: Ran from Weekly Shonen Sunday Issue 3-4 until Issue 45

And at the same time, in Champion, this story was in the middle of its long run:

Abashiri Family: Weekly Shonen Champion

There was also King magazine.

Supokon-kun: Ran From Issue 2 of Weekly Shonen King to Issue 8

So, from the beginning of 1972, while it was only for a short while, Go Nagai held the achievement of having conquered five weekly manga magazines. Pretty awesome, huh? Bet he didn't sleep. That's probably a record which will never be duplicated. (Edit: I was wrong. See "The Case of George Akiyama".)

And even though it was drawn in an environment such as this, Devilman remains to this day. (To commemorate the 40th anniversary of its debut, it's being reprinted in a collectors' edition. Due out from Kodansha in 2008, at 2000 yen plus tax, see Amazon and bk1.)